A Scalable Solar Power System - On a Budget

A friend asked me to design a solar power system that could be used to power a side by side refrigerator/freezer and a separate free-standing freezer, both for two hours each day. The initial system needed to be less than $500, and needed to be scalable; meaning additional components could be added over time. My friend had his eye on the Harbor Freight 45 watt kit, just like many preppers, however the Harbor Freight kit will not produce enough power to support his needs. So before we get into the system components, let’s first look at the requirements.

System Requirements

For our system there are several challenges and each poses certain considerations. The first challenge is making everything affordable, which sometimes competes with also making the system scalable. For instance, if you goal is to have 200 watts of solar panels, it is cheaper to buy a single 200 watt solar panel instead of purchasing two x 100 watt panels or four x 50 watt panels. Larger solar panels generally cost less per watt than smaller ones. However, purchasing a large solar panel may be outside of the initial budget for your system. Therefore, for this system build we will use 100 watt solar panels. Second, when purchasing items like a charge controller or inverter, there are many different options, different technologies, and a wide price variance; depending on your budget you may not purchase the best item for the task. Last, each system is designed to an individual’s requirements, and these requirements may not be your own; so conduct your research before purchasing items.

Now back to the system. The primary goal of this solar power system is to provide enough power to run a fridge/freezer for two hours a day, and a separate freezer also for two hours a day. So before we start purchasing components, we need to understand how much power each appliance will require. In our Alternative Energy Systems and Power Consumption article, we looked at power consumption for several different electronics. Our study showed that a large side by side refrigerator/freezer required 12 amps/hr and a chest freezer required 9 amps/hr. So, the side by side will require 24 amps each day (12 amps/hr x 2 hours) and the free-standing freezer will require 18 amps each day (9 amps/hr x 2 hours). Combined, the appliances will consume around 42 amps each day.

Our power consumption study was for specific models, so your appliances may require more or less energy. Since the amp/hr ratings above are estimates, we should also account for overages. So we will add 20% to the total, which now gives us 50.4 amps that will need to be replaced every day by our solar panels (42 amps + 8.4 = 50.4 amps).

If you want to know how much power your devices will consume, we recommend using a meter. For DC applications, we use the Watt’s Up Meter. For AC applications, we recommend a Kill A Watt Monitor. Keep in mind, if you are using an inverter to power your electronics, there will be added consumption since the inverter requires power, and there is some power loss when going between AC to DC (more on this later).

So now that we understand the requirements, let’s shift to the key components.

Solar Panels

So, how many watts of solar panels are required to produce the 50 amps needed? Our friends over at AM Solar have prepared a System Sizing guide; remember these are only estimates. A 100 watt panel produces an average of 6 amps per peak sun hour, or about 30 amp-hours per day. When planning a solar power system, the standard for peak sun hours is 5 hours a day; depending on where you live you may get more or less. Therefore, at a minimum, we will need two x 100 watt panels to replace the 54 amps used by the appliances (6 amps/hr X 5 peak sun hours x 2 panels = 60 amp-hours per day). Since the 200 watts of solar panels barely replaces the expended battery capacity, for this example, it is recommended to have another 100 watt panel, for a total of 300 watts of solar panels; as the budget allows.

In comparison, a single Harbor Freight 45 Watt solar power kit would be significantly under-sized to support our power requirements. Our system goal is a minimum of 200 watts of solar, which would require approximately 5 Harbor freight systems. At the current price of $189 per Harbor Freight Kit, it would cost $945 to get to the 200 watts of solar panels we require. In contrast, you could purchase 2 x 100 Watt panels for $300, and invest the remaining $645 in the other components. Or, for $900 you could purchase over 600 watts of panels, and still have a few dollars left over. Now, there is nothing wrong with the Harbor Freight Kits; they are just not practical if you are trying to power amp-hungry devices or require more than one kit.

Charge Controller

The charge controller regulates the charge to the batteries, and must be sized to the solar panels. For our example, a 25 amp charge controller will be adequate since it is rated for 300 watts of solar panels, or less. If you wanted to go above 300 watts of solar panels, you would require a larger charge controller (30 amp charge controller - 450 watts or less, 45 amp charge controller - 600 watts or less).

Your charge controller needs to be compatible with the overall voltage of your system. Most alternative energy systems/components are 12 volt, 24 volt, or 48 volts. 12 volt is great for smaller systems, whereas 24 volt and 48 volt are better for larger systems, grid-tied systems, when incorporating wind or hydro, or when long distances need to be covered (solar panels to batteries). Some charge controllers will accommodate varying voltages; however some will only accommodate one voltage. Therefore you must select the appropriate charge controller for your system, or one that can accommodate multiple voltages.

There are different types of charge controllers. For this article we will discuss PWM and MPPT. PWM controllers, short for Pulse-Width modulation, are the industry standard, are three stage controllers (float, bulk charge, and trickle charge), and are reasonably priced. MPPT, or Maximum Power Point Tracking, are also three stage charge controllers, cost more than PWM controllers, are 94-98% efficient, and can increase power to your battery by 10-30%. With a MPPT charge controller, let’s say your panels are producing 10 amps, which is pushed to the MPPT charge controller which regulates the charge to your batteries. If needed, the MPPT controller then pushes 10-30% more power to your batteries; so that 10 amps your panels are generating now becomes 11-13 amps to the batteries. But, with this surge in power comes a hefty price jump. So which one do you need? Either one; both are effective and will accomplish the task, that is if you purchase a quality charge controller.

Battery Capacity

The battery bank is also a vital component of any alternative energy system. Without a large battery bank, the system would be unable to run after several days of no sun or poor charging conditions. So taking the 50 amps total from above, we would want a minimum of two days of running the system without needing to recharge the batteries, or a minimum battery capacity of 100 amp/hr (AH). Additionally, you do not want to drain your batteries beyond 50%, so we will need to double the 100 AH, which means we will need a minimum of 200 AH of battery capacity (50.4 amps x 2 days = 100.8 amps x 2 = 201.6 amps).Ideally, you would want a little more buffer, so adding additional batteries would be a priority after the initial system is built.

Deep cycle batteries are recommended for alternative energy systems, since they can take more abuse (discharge). For my personal system, I use UPG 45978 100 AH batteries.Believe it or not, the batteries will cost just as much as the solar panels.

When selecting batteries, you need to determine at what voltage your system will run. Personally, I like 12 volt systems for simplicity; however 24 or 48 volt systems are more practical for larger systems. Deep cycle batteries generally come in 6 volt and 12 volt options. Depending on how you connect your batteries, parallel or in a series, will determine if your amps increase, or voltage increases, or a combination of both. If you connect in a series, you will double your voltage, but your capacity remains the same. If you connect in parallel, your voltage remains the same, but your capacity doubles.

So, if you building a 12 volt system, you would require a minimum of two x 6 volt batteries wired in series or one 12 volt battery. If you add more than one 12 volt battery, you need to wire in parallel, which will increase your battery capacity. For this system we will use 12 volt batteries wired in parallel.

Note: One way to avoid draining your batteries is running your power hungry devices when your solar panels are receiving prime sunlight. Instead of charging your batteries, the power will be fed directly to your devices.

Inverter

An invert allows batteries, which are direct current (DC), to power alternating current (AC) electronics, such as a refrigerator. Basically, it converts DC power to AC power. There are different types of inverters, but for this example we will discuss true sine wave and modified sine wave. A true sine wave inverter mimics the AC power output from your home, and is recommended if you have electronics that are sensitive to power fluctuations; such as refrigerators, televisions, air conditioners, computers, and medical equipment. While the best all-around choice, true sine wave inverters are more expensive, usually twice as expensive when compared to modified sine wave inverters. Modified sine wave inverters are more affordable, but may not always work with your electrical devices.

Next, when picking an inverter you need to determine how many watts you will need to support at a time. Most inverters are rated by continuous-watts and surge-watts. Since the individual will be running larger appliances, it is recommended to get an inverter which supports a minimum of 1500 continuous-watts.

Like we stated earlier, inverters also require power. A larger inverter, in theory, consumes more power than a smaller inverter. Therefore, if you goal is to only run a small laptop, you can get a smaller inverter. The point here is to scale your inverter to you planned usage, but also account for being able to support larger appliances. Or, you can use more than one inverter.

Last, the inverter will need to support the overall voltage of your system. So if you have a 12 volt system, you will need a 12 volt inverter, 24 volt system – a 24 volt inverter, and so on.

Wiring

Wiring is another thing that can cost a considerable amount of money. For short runs, 4 gauge wire should be acceptable (less than 5 feet). However, if you are using a large inverter and powering amp-hungry devices, you may want to go with larger wire (2, 0, or 1/0 gauge or larger). Your wire size is important when using an inverter. The trick for inverters is putting them close to the battery bank, less than 5 feet away, and running extension cords to your appliances from the inverter. If you put your inverter further than 5 feet away from you battery bank, more than likely you will get unsatisfactory results.

To wire our system, we will use a combination of 4 and 8 gauge wire, and connect everything together with a dual power post (as used in our other solar power projects). We will place a quick disconnect between the solar panels and charge controller, so that maintenance can be conducted on the system. Additionally, we will place a 200 amp fuse between the inverter and the power post.

Add-on Components - Phase II (As Budget Allows)

After the initial system is built, it is recommended to upgrade the battery capacity, followed by adding additional solar panels. Another consideration is investing in a high quality charge controller. The TriStar TS-45 and TriStar Digital Meter have been recommended by a few of my friends, since it is highly scalable. With 45 amps of wiggle room, it also supports 12-48 volt systems. While not necessary, the digital meter allows you to see what is happening with your panels, batteries, and overall system performance.

Conclusion

Our example system was designed for a friend; however it can easily be modified for anyone that is starting out with solar power. One of the reasons why we have shared this system build, is because there have been many discussions in our forum, referencing the Harbor Freight 45 watt solar power system. While the HF system could easily power LED lights, and be used with a small inverter, it is just not practical for more than that. So that leads people to purchasing additional HF systems. For their price point, the HF system will cost you a lot more money in the long run. Instead, we recommend purchasing individual components, and putting a system together on your own.

Please let us know if you have any questions, and feel free to look at our other solar power articles.